1. Field of the Invention
The present invention relates to a differential transmission circuit, an optical module, and a method of manufacturing a differential transmission circuit.
2. Description of the Related Art
For example, optical modules (optical transceiver modules) for optical fiber transmission have been increased in speed and reduced in size and cost along with the widespread use of broadband networks in recent years. In general, a differential transmission is used for high-speed digital signal transmission. Thus, on one surface of a printed circuit board of the optical module, a plurality of channels are formed of differential transmission lines (balanced transmission lines) for high-speed digital signals.
On the other hand, when the plurality of differential transmission lines are arranged close to each other on the printed circuit board, there is a fear of an influence due to forward crosstalk between the transmission lines. The forward crosstalk refers to crosstalk generated on the far end, which is opposite to a signal source, when two pairs of differential transmission lines for propagation in the same direction have a region where the differential transmission lines are arranged close and parallel to each other. The crosstalk amount of the transmission line is known to increase as the length of this region increases and as the signal bit rate increases. As described above, in the optical module that is increasingly downsized and is required to have a plurality of differential transmission lines arranged thereon, a technology for suppressing the crosstalk amount to the minimum is important. Hitherto, as a method of reducing the crosstalk amount between the differential transmission lines, in U.S. Pat. No. 7,609,125, there is disclosed a technology using metal wiring that is connected to ground wiring and arranged between and parallel to the plurality of differential transmission lines formed of strip conductor pairs, to thereby obtain an effect of reducing the crosstalk amount.